The invention relates to a false twist device for a synthetic multifilament yarn for use in false twist texturing machines.
In the case of false twist texturing machines, one starts out from the principle that, while advancing through a false twist device, the synthetic multifilament yarn is put in a twisted condition at, for example, more than 2,000 rpm, and heated in this twisted condition to the plasticization limit. Thereafter, it is again cooled to a temperature that is below the first-order transition point.
To impart a twist, a false twist device is used that is known from U.S. Pat. No. 4,235,071. The false twist device consists of three sets of disks that are each mounted on an axle for rotation in the same direction. The axles are arranged in the corners of an equilateral polygon, so that the disks overlap in the center thereof. In the center of the polygon, the yarn advances through the false twist device. In so doing, the yarn contacts the circumferential surface of the rotating disks, and is thus twisted. Heating and subsequent cooling in the false twist machine set the form predetermined by the twist impartation, so that after untwisting, a crimp remains in the yarn.
In this process, it should be taken into account that currently the false twist texturing machines process yarns of polyester, in particular, polyethylene therephthalate, polyamide 6 (Perlon) or polyamide 6.6 (Nylon). In so doing, the yarns are processed from partially oriented feed yarns. These partially oriented feed yarns are spun at a high speed and, consequently, subjected in the texturing machine only to a small draw ratio of less than 2:1 in the case of polyester, preferably less than 1.8:1 in the case of polyamides, preferably less than 1.3:1.
In this draw process, the polyester yarns are subjected in a heating zone to temperatures from 190.degree. C. to 210.degree. C., nylon yarns from 190.degree. C. to 205.degree. C., and perlon yarns to about 170.degree. C.
The technological requirement that the effective yarn temperature in the heating zone must exceed 180.degree. C., preferably 200.degree. C. results at increasing yarn speeds and with the known cooling devices in that the yarn enters into the false twist device at an overly increased temperature. Upon its entry into the false twist device, the yarn temperature may be greater than 100.degree. C.
The increased yarn advancing speeds during the false twist impartation necessitates higher yarn tensions to obtain a stable texturing process. Such yarn tensions can be realized by increasing the draw ratio, which leads again to undesired changes in the yarn properties (primarily low elongation).
It is therefore the object of the invention to further develop a false twist device of the initially described kind such that within a false twist zone in a false twist texturing machine, a stable yarn advance is realized at yarn speeds greater than 1,000 m/min, in particular above 1,200 m/min.